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IAEA Technical Meeting on
the Phenomenology, Simulation and Modelling
of Accidents in Spent Fuel Pools
Eduard YEGHOYAN
Head of Laboratory, “Armatom” Institute
Artashes HOVHANNISYAN
I Cat. Engineer, “Armatom” Institute
2-5 September 2019,
Vienna, Austria
APPROACHES TO THE MANAGEMENT OF
SEVERE ACCIDENTS IN SFP
AT ARMENIAN NPP
1
2
The nuclear energy sector of Armenia includes one nuclear power
plant - Armenian NPP, which is located in the western part of
Ararat valley, 30 km west of the capital Yerevan.
Introduction
➢ Armenian NPPconsists of two units with Soviet design WWER-440-
270 model reactor that is a version of the WWER-440-230 serial
model with special seismic considerations in the design.
➢ Unit 1 started its commercial operation in 1976 and the Unit 2 in
1980; Both units were shut down shortly after the earthquake of
December 7th, 1988;
➢ Following the completion of repair and safety upgrading activities
Unit 2, after 6.5 years of shutdown, restarted operation in 1995 and
it has been operational since then. Unit 1 remains in long-term
shutdown.
➢ The Spent Fuel Pools (SFPs) of both units are currently in
operation - the fuel discharged from the operating unit 2 reactor
core is put first in the SFP of that unit, then, after several years of
storage, is transferred to the SFP of the unit 1.
3
Plant layout
SFPs at Armenian NPP are of “at-reactor” type. The SFPs are located
close to the reactor, but outside of the containment hermetic
boundary. The pools are constructed of reinforced concrete with a
two-layer steel liner – stainless steel and carbon steel with 4mm of
thickness each one and 4mm gap between them.
The so called “Central hall” of the reactor building, to which the SFPs
are connected, is a relatively big area – width 39m, length 126m,
height 28.3 m (the Central hall is common for two units of the plant; it
is an airtight room in the upper part of the reactor building, not
designed for overpressure).
When the fuel pool is not in refueling mode, it is covered by panels
(panels don’t ensure tightness of the pool and the pool is considered
to be connected to the Central hall).
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Plant layout
5
FIG 1. Simplified layout of the Reactor building
SFP1SFP2
Plant layout
6
FIG 2. Section of the Reactor building
SFP
RefuelingPool
SFP ACCIDENT STUDY
SPENT FUEL POOL AND COOLING SYSTEM DESIGN
The SFP has rectangular form, and has 2 separate parts – main pool
and container compartment. The racks in the SFP can have 2 levels.
The lower level is used for permanent storage of the fuel assemblies,
and the upper level of racks is used temporarily - installed and used in
short term in case of full off-load of reactor core. The number of fuel
assemblies’ cells in the SFP lower racks is 372 and in the upper level -
351. The number of fuel assemblies in reactor core – 347. The pools
have also several cells for installing special containers in the SFP for
the storage of damaged or leaking FAs.
Depending on operation mode, different coolant levels are maintained
in the pool. Heat removal from SFPs is ensured by forced circulation
of the coolant through the heat exchangers of the dedicated cooling
system. The lowest level penetration of the pool is the connection of
circulation pumps suction line (pool outlet line).
7
SFP ACCIDENT STUDY
SPENT FUEL POOL AND COOLING SYSTEM DESIGN
8
FIG 3. SFP heat removal system simplified principal scheme
SF
FP
-1
SF
PP
-2
SFPHE-1SFPHE-2
Cooling waterto heat exchangers
Passage connectingwith refuelling pool
+5.47+5.67
-1.8
+1.4
+11.8
+6.0
+4.5
-1. 86-1.48
+ 7151.
+5 44.
+2 24.
Lower levelracks
Spent Fuel Assemblies
+1.03
Upper levelracks
Containercompartment
9
SFP ACCIDENT STUDY
ACCIDENT SCENARIOS
The main scope of studies of accident scenarios in SFPs is performed
within the development of plant EOPs for SFPs and corresponding
analytical justification documentation. Main two categories of accident
scenarios considered in the studies are:
1) “Loss of SFP coolant”
2) “Loss of cooling of SFP”.
“Loss of SFP coolant” scenario can take place in case of break of
cooling system tube. For the case of coolant inlet pipe break, in order
the siphon effect is “broken” there is special small size line connected
to the containment atmosphere to insure air inlet to the cooling
system in case the highest point is under vacuum.
The elevation of tube connection to the pool prevents uncovering of
fuel when only lower level racks contain spent fuel assemblies.
SFP ACCIDENT STUDY
ACCIDENT SCENARIOS
The scenario with most serious consequences - both level of racks
full of spent fuel assemblies (reactor core off-load mode) and break
of cooling system tube (pump suction line).
In such a scenario coolant level in SFP can decrease very fast till
reaching the elevation of cooling system suction tube (see figure 4 ).
Thus, the higher part of fuel assemblies in the upper level racks can
be uncovered very soon after beginning of the transient .
According to the calculations’ results, in the beginning phase of the
transient, decrease of the coolant level in SFP is very fast. The fuel
top level is uncovered in less than 5 min. From this moment increase
of fuel temperature starts at the uncovered part and takes place
significantly fast.
10
FIG 4. Change of coolant level in the SFP in the scenario of cooling system tube
break (without SFP make-up)
SFP ACCIDENT STUDY
ACCIDENT SCENARIOS
11
Elevation, m
Coolant level
Fuel top level
Outlet tube level
Time,hours
SFP ACCIDENT STUDY
ACCIDENT SCENARIOS
Quick loss of coolant ceases when the coolant level reaches the
elevation of the tube connected to the pool. Heat removal from the
pool is lost and the coolant temperature increases continuously.
Starting from this period, during about 50 minutes the coolant level is
unchanged – the loss of coolant (mainly through evaporation) is
compensated by its thermal expansion.
About 1 hour after beginning of the transient coolant temperature
reaches the boiling temperature and decrease in coolant level starts
again due to more intensive evaporation process (with no more effect
of thermal expansion of the coolant).
In parallel to the warming up of the coolant in the lower part of the
pool (below upper level racks), the rate of steam generated in the
upper level assemblies increases, thus, improving the heat removal
by steam from the uncovered part of the fuel, and starting from about
45-46 min. of the transient the fuel temperature starts to decrease
(see figure 5).
12
FIG 5. Change of fuel maximum temperature in the scenario of cooling system
tube break (without SFP make-up)
SFP ACCIDENT STUDY
ACCIDENT SCENARIOS
13
Time,hours0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0
Fuel hottest pointtemperature, C
o
1200
1000
800
600
400
200
14
SFP ACCIDENT STUDY
ACCIDENT SCENARIOS
Continuous decrease in coolant level results in less steam generation
and bigger part of fuel uncovered, i.e. bigger decay power to be
removed by steam. After 25 minutes of fuel temperature decrease, it
starts to increase again. Overheating of the fuel cladding till 1200 oC
takes place about 4.23 hours after starting of the transient.
Another accident scenario with the same initiating event and early SFP
make-up was studied. Due to significant loss of coolant through the
break the coolant level is almost the same as in the first scenario.
Some kind of “feed-and-bleed” takes place in the pool, and the mean
temperature of the coolant does not practically increase. However,
heat removal conditions for upper level rack assemblies are
deteriorated – low temperature of coolant feeding the upper level
assemblies results in low rate of steam generation and, thus, in
continuous increase in fuel temperature in the upper uncovered part
of fuel. In this 2nd scenario the conditions of fuel damage are reached
significantly earlier (less than 1.5 hours after transient beginning).
15
SFP ACCIDENT STUDY
ACCIDENT SCENARIOS
In the scenario with loss of cooling of the SFP, even in case of reactor
core full off-load, the fuel top part will be uncovered in a time longer
than 34 hours (coolant boiling starts about 2 hours and 40 minutes
after the beginning of the transient).
For the scenarios with only lower level racks containing spent fuel
assemblies the cooling system line break will not create quick change
of heat removal conditions – at least 2.5 m layer coolant inventory will
be available in the beginning phase of the transient.
Severe accidents in SFPs of ANPP are not studied yet. Draft versions
of SFP SAMGs were developed based on known general phenome-
nology of the accident progression in SFPs. Currently the model is
under development (MELCOR 1.8.6 version is used) to support
analytical justification of the strategies considered in the SAMGs.
16
SFP ACCIDENT STUDY
MANAGEMENT OF SEVERE ACCIDENT IN SFP
Armenian NPP SAMG development program was divided in 2 phases.
In phase 1 set of SAMG documentation was developed for conditions
of closed reactor and closed (tightened) containment. In the 2nd phase
(launched in 2018) draft versions of SAM guidelines were developed
based on known phenomenology of the accident progression in SFPs
and the approaches to severe accident management used by
Westinghouse Owners Group.
The main way to stop the progress of fuel damage in the SFP is
providing coolant inventory in the pool to ensure heat removal from
fuel – through evaporation in the beginning, then, if the inventory is
enough, through circulation of the coolant via cooling system.
The attempts to ensure SFP make-up from different sources and
through different ways are considered in EOPs that are used before
transition to SAMGs. If transition from EOPs to SAMGs was done, this
means the attempts were not successful.
17
SFP ACCIDENT STUDY
MANAGEMENT OF SEVERE ACCIDENT IN SFP
After the onset of fuel damage in the SFP there are at least 2 pheno-
mena that will influence the priorities of tasks to be completed within
the severe accident management:
1) release of radioactive materials from SFP to the Central hall,
2) generation of hydrogen in the SFP and its accumulation in the
Central hall.
The priorities of strategies comparing to EOPs should be changed
making main priority limitation of radioactive releases, both current
and potential.
The way of radioactivity release to the environment is the flow of
radioactive materials from SFP atmosphere to the Central hall, and
then from there directly to the environment or to the adjacent
premises in the reactor building (Central hall is not a hermetic area).
If no strategy is implemented, continuous flow of Central hall
atmosphere to the environment will take place in result of generation
of steam and incondensable gases in the SFP.
18
SFP ACCIDENT STUDY
MANAGEMENT OF SEVERE ACCIDENT IN SFP
In the current configuration of the plant, the only way for limitation of
releases is the “direction” of gas flow through the ventilation system
filters.
Although Central hall is not hermetic, it has capability to localize to
some extent the radioactive materials and limit the releases. Thus, the
protection of Central hall boundary is an important task for SAM.
Due to the design of the Central hall, burn of hydrogen even in slow
deflagration mode could result in damage of this area boundary and in
opening of ways for unobstructed propagation of radioactive materials
to the environment. Thus, formation of flammable gas mixture in
Central hall should be excluded during the progression of the severe
accident.
19
SFP ACCIDENT STUDY
MANAGEMENT OF SEVERE ACCIDENT IN SFP
The major steps considered in draft SAMGs to be implemented within
the management of severe accident in SFP are as follows:
✓ Blocking/closing ways of radioactive materials propagation from
Central hall to the environment;
✓ Organizing filtered venting of the Central hall with the purpose of
limiting the unfiltered releases of the gases from the Central hall as
well as limiting accumulation of hydrogen in the Central hall and
adjacent areas;
✓ Identifying/recovering ways for SFP make-up;
✓ Identifying and evaluating anticipated impacts of pool make-up at
the current conditions and with available means of make-up;
✓ Control of the strategy of pool make-up;
✓ Transfer of SFP heat removal to its cooling system.
20
FIG 6. Transitions from EOPs to SAMGs
SFP ACCIDENT STUDY
MANAGEMENT OF SEVERE ACCIDENT IN SFP
EOPon power( )
NO
Shutdown EOPshutdown reactor( )
SFP EOPSFP-1,2( )
SAG-1 :0 “ ”Severe accident management on SFP-2
SAG- :11 “ ”Severe accident management on SFP-1
SAG- : Severe accident management on open reactor
9 “”
DFC-2
SAMG package
DFC
Closed reactorYES
Closed containment
YES
NO
SAG-12: “”on shutdown reactor with open containment
Severe accident management
Severe accident symptomsSevere accident symptoms Severe accident symptoms
Management of severe accident in case of radioactivematerials releases through the Reactor Building Central Hall
Management of severe accident in case of radioactivematerials releases from the containment
Management of severe accident on reactor in case of
tightened containment
Transitions from EOP to SAMG
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SFP ACCIDENT STUDY
MANAGEMENT OF SEVERE ACCIDENT IN SFP
EOPs documentation includes 3 standard sets of procedures:
• EOPs on power;
• Shutdown EOPs;
• SFP EOPs.
Fragmentation of SAM guidelines was made based on anticipated
pathway of radioactivity releases:
1) releases from containment,
2) releases through reactor building Central hall.
In the first case the source of radioactivity is the reactor, and in the
second case the source of radioactivity can be the SFP of unit 1, SFP
of unit 2 as well as the reactor (open reactor or closed reactor but
open containment - configuration of the plant possible at the end of
outage during short time period).
22
SFP ACCIDENT STUDY
MANAGEMENT OF SEVERE ACCIDENT IN SFP
In ANPP SAMGs plant diagnostics during severe accident will be
ensured using 2 different diagnostics flow-charts – DFC and DFC-2.
DFC is developed for the configuration of the plant with closed reactor
and closed containment. It controls such parameters, as dose rate on
the plant site, hydrogen concentration in containment, pressure and
temperature in containment, steam generators coolant levels, primary
pressure, core exit temperature, reactor vessel failure symptoms.
DFC-2 is referenced in case of severe accident in SFP or in open
reactor or in closed reactor with open containment - in cases when the
radioactivity release will mainly take place through Central hall.
For each of these mentioned cases there is a special guideline which
will be used to mitigate the severe accident (transitioned from DFC-2).
These guidelines can be used in parallel, if needed, depending on
number of facilities containing damaged nuclear fuel.
23
SFP ACCIDENT STUDY
MAIN TASKS DEFINED FOR ANALYTICAL PART OF SFP SAMG
Recently activities for severe accident study for spent fuel pools (SFP)
and open reactor configuration started. The MELCOR model (using
MELCOR 1.8.6 version) is under development.
The SAMG developers defined a set of tasks for analytical team.
Some of these tasks are given below:
• The model must include also the ventilation systems (including the
vent stack) to ensure the modeling of filtered venting from of the
plant Central Hall. It must model also the natural draft of the vent
stack - flow of gazes without operation of the ventilators.
• The model must be detailed enough to ensure the consideration of
the possible air ingress into the assemblies from lower head.
• The model must consider (at the extent possible) the heat exchange
of steam-gas mixture with constructions and walls of the Central
Hall (to consider the steam condensation rates and its influence on
the flammability of the gas mixture and the scope of releases).
24
SFP ACCIDENT STUDY
MAIN TASKS DEFINED FOR ANALYTICAL PART OF SFP SAMG
• The model should allow to assess the efficiency of the heat removal
by air in case the lower heads of assemblies are uncovered - to
assess how it can mitigate the heating up of fuel or if it can prevent
the overheating of the fuel, what temperatures of cladding are
anticipated in case of cooling by air.
• The calculations must include scenarios without any operator
actions to assess the phenomenology of the severe accident and
different phases of its progression as well as the scenarios with
implementation of strategies considered in the guidelines.
• The calculations must reveal the conditions (decreased level of
coolant in the pool) when the failure of fuel cladding is anticipated
as well as the tendency of change of the temperature in the SFP
(thermocouples measurement values) during the heating up of the
fuel, the runaway oxidation of fuel cladding and later phases of
severe accident. Analytical part has to define the scale of tempera-
ture measurements to be sufficient for the severe accident phase.
25
SFP ACCIDENT STUDY
MAIN TASKS DEFINED FOR ANALYTICAL PART OF SFP SAMG
• The calculations must reveal the quantity (tendency of generation)
of the hydrogen that can be generated in the pool in case of
different accident scenarios as well as the tendency of hydrogen
concentration increase in Central Hall. It must reveal if flammable
mixtures can be formed in the Central Hall and the minimal
anticipated timeframes for its formation.
• The calculations must reveal the influence of radioactive aerosols
natural deposition in Central Hall .
• The calculations must reveal the efficiency of Central Hall venting –
mitigation of releases and prevention of flammable mixture
formation (particularly for the case without availability of ventilators
and use of vent stack natural draft).
• The calculations must include the reflooding of uncovered fuel.
• Calculations must justify (or disprove) the fact that the make-up of
the pool (containing uncovered overheated fuel) with bigger flow
rates will result in better results.
Thanks for your attention!
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